As is well known in the art, Joule heating (also referred to as ohmic heating or resistive heating) can cause errors in determining an ambient temperature, for instance, where the ambient temperature of a space is to be controlled. For example, Joule heating of electronic components in a thermostat can adversely affect the determination, by the thermostat, of the ambient temperature of the space in which the thermostat is located.
In the prior art, attempts to solve this problem typically involve an assumption that, for a particular product (e.g., a particular thermostat), there is a maximum offset (i.e., at a duty cycle of 100%) between a sensed or measured temperature and the actual ambient temperature. That is, in the prior art, it is assumed that, for a particular product, the maximum offset does not vary. The assumed maximum offset is typically based on test results for the product resulting from laboratory testing. In the prior art, an adjustment is made to the assumed maximum offset, to take into account the duty cycle, in an attempt to determine the actual ambient temperature by subtracting an adjusted offset from the measured temperature.
The duty cycle is taken into account because, as is well known in the art, the error in determining the actual ambient temperature is proportional to the duty cycle. Therefore, the assumed maximum offset typically is proportionally reduced, to provide the adjusted offset. In practice, this means that the maximum offset typically is adjusted by the applicable duty cycle by multiplying the maximum offset by the duty cycle, where the duty cycle is expressed as a percentage. Ultimately, the product typically is energized for what is thought to be a correct period of time, largely based on the difference between the approximate ambient temperature (as calculated based on the foregoing assumption) and the set point temperature.
Because the maximum offset is determined in the prior art in factory conditions, the environment around the product upon its installation is not taken into account. However, assuming the same maximum offset in all circumstances for a particular product can lead to significant inaccuracies, because the environment in proximity to the product significantly affects the circulation of air in the space. The failure in the prior art to take the immediate environment into account introduces a significant error into the determination of the maximum offset. As noted above, the estimated or approximate ambient temperature typically is determined by calculations based on the maximum offset. Accordingly, it can be seen that an error in the maximum offset (i.e., a maximum offset that does not take the environment around the product into account) will result in an inaccurate estimate of the ambient temperature. This has a direct impact on the ability of the prior art product to operate properly and efficiently.
In addition, the environment around the product may change significantly from time to time, e.g., when nearby furniture is moved, or if the product is moved. For example, if a large item of furniture is pushed against the product (e.g., a baseboard heater) and remains positioned against it, the furniture significantly affects the circulation of air in the space, in and around the product. In this example, because of the changed air circulation pattern, the maximum offset should be affected by the furniture pushed against the product. The effect of a change in the environment around the product may be difficult to predict. In general, because the role environment plays in the determination of an accurate maximum offset is significant, assuming a maximum offset for a particular product that fails to take the environment around the product into account leads to an inaccurate estimate of the ambient temperature, and improper and inefficient operation of the product.